Comparison of heat conduction mode and deep penetration mode by means of high-speed synchrotron X-ray videos during laser beam welding

There are two modes of laser beam welding: the heat conduction mode and the full penetration mode. The heat conduction mode is present if the intensity of the laser beam is to low to evaporate but sufficient to melt the irradiated material. This welding mode results in a very stable process without pore formation but with low welding depths. The strong evaporation in case of higher laser beam intensities leads to the formation of a vapour capillary, whose presence defines the deep penetration mode. The capillary enables high welding depths but coincides with instabillities, which may form pores in the weld seam. In order to compare the welding modes, the space- and time-resolved geometry of the capillary and the melt pool were captured during laser beam welding of aluminium with a framerate of 1000 Hz by means of high-speed synchrotron X-ray imaging. Both Experiments were conducted with a laser power of 1 kW and a welding velocity of 6 m/min. In order to weld in heat conduction mode, the intensity of the laser beam was reduced by shifting the focal position 5 mm above the surface of the sample. The crossections of the reulting weld seam are depicted below their corresponding X-ray-video.